Improve contextType docs

See confusion in https://github.com/facebook/react/issues/13969.
Specific action items:

• Clarify .contextType is available only since 16.6 (and corresponding RN/Expo release)
• Clarify you may not be seeing it due to circular deps, and best practice is to declare it in a separate file
• People sometimes get confused thinking that createContext value is the one you need to "update", we need to explain it's driven by state
• Need more coherent doc structure

首先阅读现有的 contextType 文档以及 issue 13969 中描述的困惑。明确 React 16.6 的可用性、循环依赖相关的指导，以及由状态驱动的 createContext 用法，然后将材料重新组织成更连贯的结构；文档应涵盖列出的每一项困惑。